Induction heater construction



Nov. 28, 1939. 1.. R. JACKSON ET AL INDUCTION HEATER CONSTRUCTION Filed May 11, 1958 ATTORNEY.

} Patented Nov. 28, 1939 UNITED STATES PATENT OFFICE INDUCTION HEATER CONSTRUCTION Lloyd IL Jackson and Howard W. Russell, Columbus, Ohio, assignors to Utilities Coordinated Research, Inc., New York, N. Y., a corporation of New York This invention relates to induction heaters and has for its object to provide aheater of this type having a predetermined power factor and eiilciency.

It has long been known and use has been made of the fact that where alternating current is applied to a coil wound around a cylindrical metallic container that eddy currents will be induced in the walls of the container, the heating eiiect of which can be employed for useful purposes. It has further been known that when the container is made out of a magnetic material such as iron, the size of induced currents and thereby the heating effect are much improved so long as opcrating temperatures are such that the container remains magnetic.

Devices of the type described have, in general, a low power factor and other undesirable operating characteristics. that an improvement in power factor can be secured by surrounding the magnetic core of the device with a single turn secondary winding in the form of a non-magnetic but electrically conducting sheath.

While it has been known that a non-magnetic low resistance sheath such as described above will improve the power factor of an induction heater, we have discovered that this improvement in power factor may be accompanied by certain undesirable features. The principal object of this invention is to teach the method of choosing the sheath resistance according to principles which we have discovered so that the undesirable features may be avoided.

More specifically the objects of this invention are to teach the choice of sheath resistance either for maximum electrical efficiency, or for maximum secondary power or for the best possible combination of efliciency and power factor.

Other objects and advantages will become more fully apparent as reference is had to the accompanying drawing wherein my invention is illustrated, and in which:

Fig. l is a diametric longitudinal section through an induction heater, shown for demonstrative purposes, and

Fig. 2 is an end view of the heater of Fig. 1'.

More particularly, I indicates a metallic core through which, or in which, a fluid, viscuous or solid material, to be heated may fiow or be contained. The core may vary in thickness of sidewalls to the point of being solid and may have any desired or given exterior shape, but is here illustrated as being round.

The core I may be of iron, steel or the like It has been known, however,

and is covered by a thin sheet of non-magnetic but electrically conducting material, which acts as a single turn secondary winding. It has been previously practiced to apply a coating of copper to the core I, however, as will be demonstrated, 5 this coating should have a greater electrical resistance than is offered by copper and is preferably made of a material such as brass. This sheath may be made of a thin sheet such as foil and may be wrapped and soldered or coated by electrolysis or installed by any other suitable method.

Enveloping the sheath 2 is an electrically insulating material 3 and wrapped therearound is a multiplicity of turns of wire 4 constituting the primary winding, one end of which is connected to a source of current supply 5, grounded at 6 and the other end of which is grounded at 1.

An induction heater of this type was constructed in which the primary consisted of 400 20 turns of #14 wire, the construction being such that the sheath 2 thereof could be removed and replaced by others. Thus it was possible to vary the secondary sheath resistance without changing any of the other heater constants.

Three types of experiments were conducted with this heater. In the first experiment no sheath was used and secondary heating currents were induced in the magnetic portion of the secondary. This arrangement attached to 100 volts produced a secondary heating power of only 600 watts at a power factor of .695 and an efiiciency of 95%.

In the next experiment the old practice of making the secondary resistance as low as possible was followed. A low resistance secondary sheath made from copper was used for this experiment and the arrangement produced 3220 watts of heating power at a power factor of 0.895 and an efficiency of 71%. Thus, as older practice predicted, the heating power was increased and the power factor much improved by the use of the sheath, however, the efliciency is low.

In the third experiment a brass sheath was used having its resistance chosenaccording to our principle of combining high efficiency and high power factor. This sheath had a resistance about 10 times as great as that used in the second experiment. This arrangement produced a 50 secondary heating power of 1900 Watts with an efliciency of 89.5% and a power factor of 0.97. These results indicate a substantial improvement in power factor and efiiciency by the use of a relatively high resistance sheath-a result contrary to that which would be predicted by the teachings of previous investigators.

As a result of numerous experiments and calculations the following conclusions were established.

To obtain maximum electrical efiiciency from an induction heater we have discovered that the secondary resistance should be chosen according to the equation where T2 is the resistance of the sheath and wL: is the secondary inductive reactance.

To obtain maximum secondary power from a heater regardless of the efliciency and power factor the sheath resistance should be chosen according to the equation is the ratio of primary inductive reactance to primary resistance and the coupling factor=1.

To construct a heater which will exhibit the best possible combination of efliciency and power factor-that is, high efliciency and high power factor the sheath resistance should be chosen according to the equation WLg (EL To construct a heater which will provide the highest secondary heating power at the best efficiency possible the secondary resistance should be chosen according to the equation where as the arrangement fulfills the condition that current flowing in the part designated as the primary will induce heating currents in the part designated as the secondary and the secondary is composed of a magnetic and a non-magnetic portion.

What is claimed is:

1. In an induction heater, a primary portion and a secondary portion, said secondary portion comprising a magnetic portion and a non-magnetic portion, the resistance of said non-magnetic portion being substantially equal to the inductive reactance of said secondary portion.

2. In an induction heater, 2, primary portion and a secondary portion, said secondary portion comprising a magnetic portion and a non-magnetic portion, said non-magnetic portion being composed of brass and having an electrical resistance substantially equal to the inductive reactance of said secondary portion.

3. An induction heater composed of a core of magnetic material, a primary winding around said core, and connected to a source of electric current'supply, and a single turn secondary winding between said core and said primary winding, said secondary winding being in the form of a non-magnetic electrically conductive sheath and having a resistance substantiallyequal to the secondary inductive reactance divided by the square root of the ratio of the primary inductive reactance and the primary resistance squared plus 1.

4. An induction heater comprising a primarywinding and a secondary winding, said secondary winding being composed of non-magnetic, electrically conductive material and having a resistance substantially equal to the secondary inductive reactance divided by the cube root of the ratio of primary inductive reactance to primary resistance.

5. An induction heater composed of a comet magnetic material, a primary winding around said core, and connected to a source of electric current supply, and a single turn secondary winding between said core and said primary winding, said secondary winding being in the form of a non-magnetic electrically conductive sheath and having a resistance substantially equal to the secondary inductive reactance divided by the cube root of the ratio of Primary inductive reactance to primary resistance.

6. The method of designing the secondary of an induction heater for maximum electrical efllciency comprising a magnetic and a nommagnetic portion which consists in selecting the resistance of said non-magnetic portion such that it is substantially equal to the reactance of said secondary.

7. The method of designing for maximum secondary power an induction heater having a secondary composed of a magnetic and non-magnetic portion which consists in selecting the resistance of said non-magnetic portion according to the formula where m is the resistance of said non-magnetic portion, 1.0L: is the secondary inductive reactance,

where r: is the resistance of said non-magnetic portion, wLz is the secondary inductive reactance and is the ratio of the primary inductive reactance to primary resistance.

LLOYD R. JACKSON. HOWARD -W. RUSSELL. 

